Especially with respect to valves, but also with respect to other aggregates of vehicles, it is necessary to provide a screw connection between a pipe and a plastic sleeve of the housing of the valve. In the manufacture of housings of valves plastic is used, which for example is extruded in a mould to form the desired shape of the housing. Forming a thread is necessary for a screw connection of a pipe or conduit, whereby the positioning of the thread directly in the plastic sleeve of the housing often does not fulfill the necessary properties, especially the resistance of tear out. Therefore, it is known to use threaded inserts made from metal with respect to plastic sleeves of housings made of plastic in order to locate the thread in the metal insert.
A connection piece of the type mentioned above is known from DE 34 43 079 A1. The housing consists of plastic and comprises a plastic sleeve or an equivalent part made from the plastic of the housing, whereby a threaded plug made of metal is arranged in the plastic sleeve. The threaded plug is fixedly located in the plastic sleeve. The insert containing the thread is to be positioned in the mould for the manufacture of the housing and is firmly anchored by the plastic extruded into the mould. The metal insert ends in the end wall of the plastic sleeve, thus, there is no possibility for the supply of a tool at the insert. Such a supply is not intended because the insert is already firmly arranged in the plastic sleeve. In connecting a pipe the fitting of the pipe is screwed into the threaded insert of the sleeve. Normally, a seal must be compressed during this screwing operation. This seal is located in the region of the end wall of the plastic sleeve and seals the gap between the insert and the plastic sleeve. Compressing this axial seal during the screwing operation subjects the threaded insert to a torsional stress. When embedding the insert during the extrusion of the plastic, a stress limit is set in the arrangement of the insert in the plastic sleeve, which must not be exceeded during the mounting of the pipe. However, this stress limit of the plastic often is exceeded if the mounting of the pipe is made without the necessary sensitivity. On the other hand it is possible that the seal does not fulfill its sealing function if the stress limit is not exceeded, making it necessary to increase the torsional forces in order to reach the pressure sealing conditions. There is the danger of loosening the insert in the plastic sleeve of the housing. In this case the housing is defective and cannot be used any more. It is neither possible to compress the seal more nor to unscrew the fitting. In addition, there is the danger of damaging the plastic sleeve, if the mounting of a pipe is performed without sufficient sensitivity. The plastic sleeve gets radially extending tears. The connection cannot be used.